1. Field of the Invention
The present invention relates to a distance-measuring apparatus for measuring the distance to an object and, more particularly, to an active distance-measuring apparatus suitably used in cameras and other equipment.
2. Related Background Art
The active distance-measuring apparatus used in the cameras etc. is arranged to project light from an infrared-emitting diode (hereinafter referred to as xe2x80x9cIREDxe2x80x9d) toward the measured object, to receive reflected light of the projected light by a position sensing device (hereinafter referred to as xe2x80x9cPSDxe2x80x9d), to process a signal from this PSD by a signal processing circuit and an arithmetic circuit to output distance information, and to determine the distance to the measured object by a CPU. Since the distance measurement with only one projection of light can cause an error, it is desirable that a plurality of light projection operations be carried out to gain a plurality of distance information pieces and that the plurality of distance information pieces be integrated in a fixed period by an integrating circuit to be averaged. The integral of distance information by the integrating circuit is carried out by applying a reference voltage to an integrating capacitor to store charge therein and discharging the capacitor according to the distance information from that state.
There is, however, a possibility that the distance-measuring apparatus described above fails to carry out accurate distance measurement. Namely, in cases wherein the discharge time of the integrating capacitor is changed depending upon the distance measurement conditions etc., discharge amounts vary depending upon the difference in the discharge time. Therefore, the capacitance of the integrating capacitor can be utilized fully as long as the discharge amount is large. However, if the discharge amount is small, the integral operation cannot be performed by fully utilizing the capacitance of the integrating capacitor, and the measurement accuracy will not be always sufficient.
Therefore, the present invention has been accomplished in view of this point and an object of the present invention is to provide a distance-measuring apparatus that can measure the distance with improved measurement accuracy.
A distance-measuring apparatus according to the present invention is a distance-measuring apparatus comprising: light projecting means for projecting pulses of light toward a measured object; light receiving means for receiving reflected light of the light projected toward the measured object, at a photoreceptive position on a position sensing device according to a distance to the measured object and outputting a signal according to the photoreceptive position; arithmetic means for carrying out an arithmetic operation based on the signal outputted from the light receiving means and outputting an output ratio signal according to the distance to the measured object; integrating means comprising an integrating capacitor, the integrating means carrying out a first integral in which the signal outputted from the arithmetic means is integrated by discharging/charging the integrating capacitor according to the signal outputted from the arithmetic means and thereafter carrying out a second integral by charging/discharging the integrating capacitor at a constant current, the integrating means comparing a voltage of the integrating capacitor with a reference voltage during the second integral and outputting a comparison result signal according to a result of the comparison; and detecting means for detecting the distance to the measured object, based on the signal outputted from the integrating means; wherein the integrating means is arranged in such a manner that a capacitance of the integrating capacitor is set so as to discharge/charge the integrating capacitor to the maximum when a time necessary for the first integral is minimum, and wherein the integrating means repeatedly carries out the first integral and the second integral of the integrating capacitor when the time necessary for the first integral is longer than the minimum time, in detection of the distance to the measured object.
Another distance-measuring apparatus according to the present invention is a distance-measuring apparatus comprising: light projecting means for projecting pulses of light toward a measured object; light receiving means for receiving reflected light of the light projected toward the measured object, at a photoreceptive position on a position sensing device according to a distance to the measured object and outputting a signal according to the photoreceptive position; arithmetic means for carrying out an arithmetic operation based on the signal outputted from the light receiving means and outputting an output ratio signal according to the distance to the measured object; integrating means comprising an integrating capacitor, the integrating means carrying out a first integral in which the signal outputted from the arithmetic means is integrated by discharging or charging the integrating capacitor according to the signal outputted from the arithmetic means and thereafter carrying out a second integral by charging/discharging the integrating capacitor at a constant current, the integrating means comparing a voltage of the integrating capacitor with a reference voltage during the second integral and outputting a comparison result signal according to a result of the comparison; and detecting means for detecting the distance to the measured object, based on the signal outputted from the integrating means; wherein the integrating means comprises a plurality of integrating capacitors of different capacitances and wherein the integrating means carries out the first integral and the second integral while selecting one of the plurality of integrating capacitors so as to discharge/charge the integrating capacitor to the maximum in detection of the distance to the measured object.
Still another distance-measuring apparatus according to the present invention is a distance-measuring apparatus comprising: light projecting means for projecting pulses of light toward a measured object; light receiving means for receiving reflected light of the light projected toward the measured object, at a photoreceptive position on a position sensing device according to a distance to the measured object and outputting a signal according to the photoreceptive position; arithmetic means for carrying out an arithmetic operation based on the signal outputted from the light receiving means and outputting an output ratio signal according to the distance to the measured object; integrating means comprising an integrating capacitor, the integrating means carrying out a first integral in which the signal outputted from the arithmetic means is integrated by discharging/charging the integrating capacitor according to the signal outputted from the arithmetic means and thereafter carrying out a second integral by charging or discharging the integrating capacitor at a constant current, the integrating means comparing a voltage of the integrating capacitor with a reference voltage during the second integral and outputting a comparison result signal according to a result of the comparison; and detecting means for detecting the distance to the measured object, based on the signal outputted from the integrating means; wherein the integrating means comprises a plurality of current sources of different output current values and wherein the integrating means carries out the first integral while selecting one of the current sources so as to discharge/charge the integrating capacitor to the maximum in detection of the distance to the measured object.
The present invention can maximize the utilization of the dischargeable or chargeable dynamic range of the integrating capacitor on the occasion of the first integral. Therefore, improvement can be made in the distance measurement accuracy.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.